Methods of cracking saturated hydrocarbon feedstocks including naphthenes are well known. One of the sources of the feedstock for this steam cracking process is from a crude oil distillate. Generally, the crude oil distillate is fractionated and the lower boiling components are compressed initially and then de-compressed when a gaseous fraction comprising predominantly C1-C4 saturated hydrocarbons is released. These gases are then usually separated into a fraction comprising C1-C3 hydrocarbons predominating in ethane and propane with significant amounts of methane, and C4 hydrocarbons comprising predominantly butanes. The C1-C3 fraction is fed directly to a steam cracker in order to generate valuable ethylene and small amounts of propylene. This was usually done in the belief that normally, when cracking mixtures of paraffinic hydrocarbons, the conversion of the easier to crack component in the mixture is suppressed and the conversion of the harder to crack component is enhanced. Furthermore, the amount of ethane (sourced usually from `dry gas`, fresh ethane and recycled ethane) in such feedstock is variable due to the difficulty in storing the same when compared with the availability of propane which can be readily stored (sourced usually from LPG), and since it is not feasible to shut down a bank of crackers when the ethane availability is low. Therefore, ethane and propane have always been cracked as a mixed feedstock.
Recent studies have shown that contrary to the conventionally understood `rules`, the presence of propane in a mixed feedstock comprising propane and ethane significantly suppresses the cracking of ethane in such mixed feedstock thereby not only reducing the amount of ethylene formed in the process but also increasing the amount of ethane which has to be recycled in the process.